In the present proposal, techniques will be applied towards a transgenic analysis of brown adipose tissue (BAT) function. BAT is distinguished by its unique ability to convert metabolic fuels to heat. This property is attributed to its unusual mitochondrial proton transporter, the uncoupling protein (UCP). Because of this capability, BAT is uniquely situated to play a central role in the pathogenesis of obesity. Previous studies have attempted to examine this possibility by correlating altered states of energy balance with altered BAT metabolism. Unfortunately, these studies have generated somewhat conflicting results. In this proposal, two alternative approaches are put forward. The first will use gene targeting in embryonic stem (ES) cells to disrupt the UCP gene. These targeted ES cells will be used to produce UCP deficient mice. The second approach will utilize suicide DNA vectors consisting of a toxin cDNA whose expression is driven by the UCP promoter/enhancer to specifically ablate BAT in transgenic mice. These complementary approaches should provide new insight into the role of BAT in the regulation of energy balance.